1. Field of the Invention
The invention pertains to the field of variable camshaft timing (VCT) systems. More particularly, pertains to a dual dependent VCT system, wherein desired intake global phase is reached by direct controlling the global phase of exhaust VCT phaser and controlling the local phase of intake VCT phaser.
2. Description of Related Art
Internal combustion engines have become increasingly complex, as features such as variable cam timing (VCT) and active noise cancellation are included. For example, using VCT, the angular displacement, or phase of a camshaft, relative to the crankshaft to which it is drivably connected, is dynamically altered to bring about changes in engine characteristics, such as fuel economy, power, or emission. Typically, there is a feedback loop in which the desired values of such engine characteristics are measured against their existing values, and changes are effected inside the engine in response to discrepancies. To accomplish this, modern automobiles usually have a control module (or more than one) having a microcomputer that constantly analyzes data fed into it from various parts of the engine and other parts of the automobile and ambient conditions (exhaust gas sensors, pressure and temperature sensors, etc.) and emits signals in response to such data. For example, in regard to VCT, as changes occur in engine and external conditions, the angular displacement between the cam shaft and the crank shaft that drives it is altered.
Referring to FIG. 1, a prior art feedback loop 10 is shown. The control objective of feedback loop 10 is to have a spool valve in a null position. In other words, the objective is to have no fluid flowing between two fluid holding chambers of a phaser (not shown) such that the VCT mechanism at the phase angle given by a set point 12 with the spool 14 stationary in its null position. This way, the VCT mechanism is at the correct phase position and the phase rate of change is zero. A control computer program product which utilizes the dynamic state of the VCT mechanism is used to accomplish the above state.
The VCT closed-loop control mechanism is achieved by measuring a camshaft phase shift xcex80 16, and comparing the same to the desired set point r 12. The VCT mechanism is in turn adjusted so that the phaser achieves a position which is determined by the set point 12. A control law 18 compares the set point 12 to the phase shift xcex80 16. The compared result is used as a reference to issue commands to a solenoid 20 to position the spool 14. This positioning of spool 14 occurs when the phase error (the difference between set point r 12 and phase shift 20) is non-zero.
The spool 14 is moved toward a first direction (e.g. right) if the phase error is negative (retard) and to a second direction (e.g. left) if the phase error is positive (advance). It is noted that the retarding with current phase measurement scheme gives a larger value, and advancing yields a small value. When the phase error is zero, the VCT phase equals the set point r 12 so the spool 14 is held in the null position such that no fluid flows within the spool valve.
Camshaft and crankshaft measurement pulses in the VCT system are generated by camshaft and crankshaft pulse wheels 22 and 24, respectively. As the crankshaft (not shown) and camshaft (also not shown) rotate, wheels 22, 24 rotate along with them. The wheels 22, 24 possess teeth which can be sensed and measured by sensors according to measurement pulses generated by the sensors. The measurement pulses are detected by camshaft and crankshaft measurement pulse sensors 22a and 24a, respectively. The sensed pulses are used by a phase measurement device 26. A measurement phase difference is then determined. The phase between a cam shaft and a crankshaft is defined as the time from successive crank-to-cam pulses, divided by the time for an entire revolution and multiplied by 360. degree. The measured phase may be expressed as xcex80 16. This phase is then supplied to the control law 18 for reaching the desired spool position.
A control law 18 of the closed-loop 10 is described in U.S. Pat. No. 5,184,578 and is hereby incorporate herein by reference. A simplified depiction of the control law is shown in FIG. 2. Measured phase 26 is subjected to the control law 18 initially at block 30 wherein a Proportional-Integral (PI) process occurs. PI process is the sum of two sub-processes. The first sub-process includes amplification; and the second sub-process includes an integration. Measured phase is further subjected to phase compensation at block 32, where control signal is adjusted to increase the overall control system stability before it is sent out to drive the actuator, in the instant case, a variable force solenoid.
To avoid confusion, the following two terms, global phase and local phase, are introduced. Global phase is defined as the relative angular position for both the intake and exhaust VCT phasers with respect to crankshaft. Local phase is defined as the relative angular position for only the intake VCT phaser with respect to exhaust VCT phaser.
A cam phaser control method, which is described in U.S. Pat. No. 5,184,578, is hereby incorporated herein by reference, describes a negative feedback loop. As can be appreciated, the loop is analogous to FIGS. 1 and 2. The loop is briefly described here merely to incorporate the concept of global and local phases respectively. The exhaust global set point is passed through a set point filter and compared with the measured exhaust global phase. The difference is then passed through a PI controller and a phase compensator. The calculated value is then added by a null value. The final result is the control value to be sent to either a PWM driving circuit or a current driving circuit to move the control actuator.
The performance of an internal combustion engine can be improved by the use of dual camshafts, one shaft to operate the intake valves of the various cylinders of the engine and the other shaft to operate the exhaust valves. Typically, one of such camshafts is driven by the crankshaft of the engine, through a sprocket and chain drive or a belt drive, and the other of such camshafts is driven by the first, through a second sprocket and chain drive or a second belt drive. Alternatively, both camshafts can be driven by a single crankshaft powered chain drive or belt drive. Engine performance in an engine with dual camshafts can be further improved, in terms of idle quality, fuel economy, reduced emissions or increased torque, by changing the positional relationship of one of the camshafts, usually the camshaft which operates the intake valves of the engine, relative to the other camshaft and relative to the crankshaft, to thereby vary the timing of the engine in terms of the operation of intake valves relative to its exhaust valves or in terms of the operation of its valves relative to the position of the crankshaft.
It is desirous, therefore to provide a dual dependent VCT system, wherein desired intake global phase is reached by direct controlling the global phase of exhaust VCT phaser and controlling the local phase of intake VCT phaser.
A system and method are provided using a pair of dual dependent cam shafts to improve feed back control.
A system and method are provided by directly controlling the global phase of exhaust VCT phaser and controlling the local phase of intake VCT phaser, the desired intake global phase is reached and a desired control signal created.
Accordingly, a variable cam timing (VCT) control system used in an internal combustion engine with the system having a dual dependent cam shaft configuration is provided. In the dual dependent cam shaft configuration, an intake cam shaft is dependent upon an exhaust cam shaft. The control system includes: a) an exhaust phaser engaging the exhaust cam shaft; b) an intake phaser engaging the intake cam shaft, the movement of the intake cam shaft being dependent upon the movement of the exhaust cam shaft; c) a first feedback loop for correcting errors relating to the intake phaser, the first feedback loop including a measured intake phase signal, being used to compare with a local intake set point and being used to generate an error signal used by the first feedback loop; and d) a second feedback loop for correcting errors relating to the exhaust phaser, the second feedback loop including a measured exhaust phase signal.
Accordingly, in a variable cam timing (VCT) control system used in an internal combustion engine, with the system having a dual dependent cam shaft configuration is provided. In the dual dependent cam shaft configuration, an intake cam shaft is dependent upon an exhaust cam shaft. A method comprising the steps is provided. The steps includes: providing an exhaust phaser engaging the exhaust cam shaft; providing an intake phaser engaging the intake cam shaft, the movement of the intake cam shaft being dependent upon the movement of the exhaust cam shaft; providing a first feedback loop for correcting errors relating to the intake phaser, the first feedback loop including a measured intake phase signal, being used to compare with a local intake set point and being used to generate an error signal used by the first feedback loop; providing a second feedback loop for correcting errors relating to the exhaust phaser, the second feedback loop including a measured exhaust phase signal; and using the measured exhaust phase signal, correcting both a global intake set point and an exhaust set point, thereby providing a more accurate correction to the dual dependent variable cam timing system.